Add i686-unknown-uefi target

This adds a new rustc target-configuration called 'i686-unknown_uefi'.
This is similar to existing x86_64-unknown_uefi target.

The i686-unknown-uefi target can be used to build Intel Architecture
32bit UEFI application. The ABI defined in UEFI environment (aka IA32)
is similar to cdecl.

We choose i686-unknown-uefi-gnu instead of i686-unknown-uefi to avoid
the intrinsics generated by LLVM. The detail of root-cause and solution
analysis is added as comment in the code.
For x86_64-unknown-uefi, we cannot use -gnu, because the ABI between
MSVC and GNU is totally different, and UEFI chooses ABI similar to MSVC.
For i686-unknown-uefi, the UEFI chooses cdecl ABI, which is same as
MSVC and GNU. According to LLVM code, the only differences between MSVC
and GNU are fmodf(f32), longjmp() and TLS, which have no impact to UEFI.
As such, using i686-unknown-uefi-gnu is the simplest way to pass the build.

Adding the undefined symbols, such as _aulldiv() to rust compiler-builtins
is out of scope. But it may be considered later.

The scope of this patch is limited to support target-configuration.

No standard library support is added in this patch. Such work can be
done in future enhancement.

Cc: Josh Triplett <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

Pkgsrc changes:
 * Remove patch which no longer applies (but what about RPATH?)
 * Adapt a few patches to changed files upstream.

Upstream changes:

Version 1.39.0 (2019-11-07)
===========================

Language
--------
- [You can now create `async` functions and blocks with `async fn`,
  `async move {}`, and `async {}` respectively, and you can now call
  `.await` on async expressions.][63209]
- [You can now use certain attributes on function, closure, and function
  pointer parameters.][64010] These attributes include `cfg`, `cfg_attr`,
  `allow`, `warn`, `deny`, `forbid` as well as inert helper attributes used
  by procedural macro attributes applied to items. e.g.
  ```rust
  fn len(
      #[cfg(windows)] slice: &[u16],
      #[cfg(not(windows))] slice: &[u8],
  ) -> usize {
      slice.len()
  }
  ```
- [You can now take shared references to bind-by-move patterns in the
  `if` guards of `match` arms.][63118] e.g.
  ```rust
  fn main() {
      let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);

      match array {
          nums
  //      ---- `nums` is bound by move.
              if nums.iter().sum::<u8>() == 10
  //                 ^------ `.iter()` implicitly takes a reference to `nums`.
          => {
              drop(nums);
  //          ----------- Legal as `nums` was bound by move and so we have ownership.
          }
          _ => unreachable!(),
      }
  }
  ```

Compiler
--------
- [Added tier 3\* support for the `i686-unknown-uefi` target.][64334]
- [Added tier 3 support for the `sparc64-unknown-openbsd` target.][63595]
- [rustc will now trim code snippets in diagnostics to fit in your terminal.]
  [63402] **Note** Cargo currently doesn't use this feature. Refer to
  [cargo#7315][cargo/7315] to track this feature's progress.
- [You can now pass `--show-output` argument to test binaries to print the
  output of successful tests.][62600]

\* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

Libraries
---------
- [`Vec::new` and `String::new` are now `const` functions.][64028]
- [`LinkedList::new` is now a `const` function.][63684]
- [`str::len`, `[T]::len` and `str::as_bytes` are now `const` functions.][63770]
- [The `abs`, `wrapping_abs`, and `overflowing_abs` numeric functions are
  now `const`.][63786]

Stabilized APIs
---------------
- [`Pin::into_inner`]
- [`Instant::checked_duration_since`]
- [`Instant::saturating_duration_since`]

Cargo
-----
- [You can now publish git dependencies if supplied with a `version`.]
  [cargo/7237]
- [The `--all` flag has been renamed to `--workspace`.][cargo/7241] Using
  `--all` is now deprecated.

Misc
----
- [You can now pass `-Clinker` to rustdoc to control the linker used
  for compiling doctests.][63834]

Compatibility Notes
-------------------
- [Code that was previously accepted by the old borrow checker, but rejected by
  the NLL borrow checker is now a hard error in Rust 2018.][63565] This was
  previously a warning, and will also become a hard error in the Rust 2015
  edition in the 1.40.0 release.
- [`rustdoc` now requires `rustc` to be installed and in the same directory to
  run tests.][63827] This should improve performance when running a large
  amount of doctests.
- [The `try!` macro will now issue a deprecation warning.][62672] It is
  recommended to use the `?` operator instead.
- [`asinh(-0.0)` now correctly returns `-0.0`.][63698] Previously this
  returned `0.0`.

[62600]: rust-lang/rust#62600
[62672]: rust-lang/rust#62672
[63118]: rust-lang/rust#63118
[63209]: rust-lang/rust#63209
[63402]: rust-lang/rust#63402
[63565]: rust-lang/rust#63565
[63595]: rust-lang/rust#63595
[63684]: rust-lang/rust#63684
[63698]: rust-lang/rust#63698
[63770]: rust-lang/rust#63770
[63786]: rust-lang/rust#63786
[63827]: rust-lang/rust#63827
[63834]: rust-lang/rust#63834
[63927]: rust-lang/rust#63927
[63933]: rust-lang/rust#63933
[63934]: rust-lang/rust#63934
[63938]: rust-lang/rust#63938
[63940]: rust-lang/rust#63940
[63941]: rust-lang/rust#63941
[63945]: rust-lang/rust#63945
[64010]: rust-lang/rust#64010
[64028]: rust-lang/rust#64028
[64334]: rust-lang/rust#64334
[cargo/7237]: rust-lang/cargo#7237
[cargo/7241]: rust-lang/cargo#7241
[cargo/7315]: rust-lang/cargo#7315
[`Pin::into_inner`]: https://doc.rust-lang.org/std/pin/struct.Pin.html#method.into_inner
[`Instant::checked_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_duration_since
[`Instant::saturating_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.saturating_duration_since

- aarch64-unknown-none: rust-lang/rust#68334
- aarch64-unknown-none-softfloat: rust-lang/rust#64589
- armv7-unknown-linux-gnueabi: rust-lang/rust#63107
- armv7-unknown-linux-musleabi: rust-lang/rust#63107
- aarch64-apple-tvos: rust-lang/rust#68191
- armv7a-none-eabihf: rust-lang/rust#68253
- i686-unknown-uefi: rust-lang/rust#64334
- thumbv7neon-unknown-linux-musleabihf: rust-lang/rust#66103
- x86_64-apple-ios-macabi: rust-lang/rust#63467
- x86_64-apple-tvos: rust-lang/rust#68191
- x86_64-linux-kernel: rust-lang/rust#64051
- x86_64-unknown-hermit-kernel: rust-lang/rust#66713
- x86_64-unknown-illumos: rust-lang/rust#71145

Moved to tier 2:
- aarch64-unknown-none: rust-lang/rust#68334